1. Field of the Invention
The present invention relates to a sheet post-processing apparatus with an improved mechanism for conveying a sheet from a processing tray to a stacking tray and a sheet post-processing method.
2. Description of the Related Art
In a sheet post-processing apparatus, a leading end of a sheet stacked on a processing tray is in contact with an upper surface of a stacking tray or an upper surface of sheets already stacked on the stacking tray. When the sheet stacked on the processing tray is conveyed to the stacking tray in this state, static friction occurs between the conveyed sheet and the upper surface of the stacking tray or the upper surface of the sheets already stacked on the stacking tray. A coefficient of static friction is larger than a coefficient of dynamic friction. Therefore, the leading end of the sheet in contact with the stacking tray hardly moves. On the other hand, other portions of the sheet easily move. As a result, the center of the sheet starts to bend upward (see FIGS. 15 and 16). When the sheet is fed to the stacking tray while bending upward, the sheet is stacked on the stacking tray in a nonaligned state. When stacked plural sheets are conveyed, a leading end of a sheet on a lower side hangs down (curves to the lower side) because the leading end of the sheet on the lower side is not supported by the processing tray. When the sheets are conveyed in this state, the sheets on the lower side among the plural stacked sheets are stacked on the stacking tray in a state in which the leading end side thereof is bent inward.
Japanese Patent Disclosure (Kokai) No. 2004-284773; Y. Takaishi et al.; Oct. 14, 2004 discloses a sheet discharging apparatus that can discharge, even when a swell, curl, or the like due to staples is present in a sheet 90 discharged onto a discharge tray 49, the sheet 90 onto the discharge tray 49 in a good posture. The sheet discharging apparatus includes a sheet tray 46 onto which sheets having images formed thereon are discharged in a stacked state and plural pressing members 55 that press a trailing end in a discharging direction of the sheets discharged onto the sheet tray 46. The plural pressing members 55 are arranged along a direction orthogonal to a discharging direction of the sheets and urged to individually press the sheets discharged onto the discharge tray 49.